linux-kernel - Re: [PATCH v10 02/28] x86/fpu/xstate: Modify the initialization helper to handle both static and dynamic buffers

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Message-ID: <5767911A-6F1F-4EDE-92A4-D4C3E5A3AABF@intel.com>
Date:   Sun, 3 Oct 2021 22:35:25 +0000
From:   "Bae, Chang Seok" <chang.seok.bae@...el.com>
To:     Thomas Gleixner <tglx@...utronix.de>
CC:     "bp@...e.de" <bp@...e.de>, "Lutomirski, Andy" <luto@...nel.org>,
        "mingo@...nel.org" <mingo@...nel.org>,
        "x86@...nel.org" <x86@...nel.org>,
        "Brown, Len" <len.brown@...el.com>,
        "lenb@...nel.org" <lenb@...nel.org>,
        "Hansen, Dave" <dave.hansen@...el.com>,
        "Macieira, Thiago" <thiago.macieira@...el.com>,
        "Liu, Jing2" <jing2.liu@...el.com>,
        "Shankar, Ravi V" <ravi.v.shankar@...el.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>
Subject: Re: [PATCH v10 02/28] x86/fpu/xstate: Modify the initialization
 helper to handle both static and dynamic buffers

On Oct 1, 2021, at 05:45, Thomas Gleixner <tglx@...utronix.de> wrote:
> On Wed, Aug 25 2021 at 08:53, Chang S. Bae wrote:
>> Have the function initializing the XSTATE buffer take a struct fpu *
>> pointer in preparation for dynamic state buffer support.
>> 
>> init_fpstate is a special case, which is indicated by a null pointer
>> parameter to fpstate_init().
>> 
>> Also, fpstate_init_xstate() now accepts the state component bitmap to
>> customize the compacted format.
> 
> That's not a changelog. Changelogs have to explain the WHY not the WHAT.
> 
> I can see the WHY when I look at the later changes, but that's not how
> it works.

The same feedback was raised before [1]. I thought this changelog has been
settled down with Boris [2].

How about:

    “To prepare dynamic features, change fpstate_init()’s argument to a struct
     fpu * pointer instead of a struct fpregs_state * pointer.  A struct fpu
     will have new fields to handle dynamic features."

With fpstate_init_xstate() changes in a separate patch and defining init_fpu,
the last two sentences shall be removed.

> Also the subject of this patch is just wrong. It does not make the
> functions handle dynamic buffers, it prepares them to add support for
> that later.

How about “Prepare fpstate_init() to handle dynamic features"

>> +static inline void fpstate_init_xstate(struct xregs_state *xsave, u64 mask)
>> +{
>> +	/*
>> +	 * XRSTORS requires these bits set in xcomp_bv, or it will
>> +	 * trigger #GP:
>> +	 */
>> +	xsave->header.xcomp_bv = XCOMP_BV_COMPACTED_FORMAT | mask;
>> +}
> 
> This wants to be a separate cleanup patch which replaces the open coded
> variant here:

Okay, maybe the change becomes to be the new patch1.

>> diff --git a/arch/x86/kernel/fpu/xstate.c b/arch/x86/kernel/fpu/xstate.c
>> index fc1d529547e6..0fed7fbcf2e8 100644
>> --- a/arch/x86/kernel/fpu/xstate.c
>> +++ b/arch/x86/kernel/fpu/xstate.c
>> @@ -395,8 +395,7 @@ static void __init setup_init_fpu_buf(void)
>> 	print_xstate_features();
>> 
>> 	if (boot_cpu_has(X86_FEATURE_XSAVES))
>> -		init_fpstate.xsave.header.xcomp_bv = XCOMP_BV_COMPACTED_FORMAT |
>> -						     xfeatures_mask_all;
>> +		fpstate_init_xstate(&init_fpstate.xsave, xfeatures_mask_all);

Thanks,
Chang

[1] https://lore.kernel.org/lkml/20201207171251.GB16640@zn.tnic/
[2] https://lore.kernel.org/lkml/20210115124038.GA11337@zn.tnic/